This invention relates to broadband (e.g., cable TV) transmission systems.
Typical broadband transmission systems broadcast TV signals over a coaxial cable from a head end. The signals are tapped off at a pole or pedestal near a customer's premises and a drop cable is run from the tap to the customer or to an interface device on the side of the customer's residence. Since the coaxial cable usually includes all channels, it is desirable to provide, at some point, interdiction circuitry which will scramble the channels not paid for by the subscriber. In many systems, it is desirable to provide this circuitry in the interface device. This type of system presents a problem in security since the drop cable will include all channels, and a subscriber may attempt to by-pass the interdiction circuitry in order to receive programs not paid for.
Several proposals have been made to prevent tampering by the subscriber. For example, U.S. Pat. No. 4,443,815 issued to Hempell describes a circuit which detects the interruption of a dc signal path through the center conductor of a cable coupled to the subscriber's set in the event the subscriber attempts to disconnect the cable. A switch is then operated by a relay to cut off any further signals from the drop cable. While such a scheme is effective, it would be more desirable not to rely on the center conductor of the cable for detection since added impedances on the conductor can adversely affect insertion and return losses in signal transmission. In U.S. Pat. No. 5,331,412 issued to Farmer et al., service is cut off to a subscriber by a relay in a circuit at the tap if signals added to the standard video signals are interrupted. The relay is controlled by a microprocessor which is also at the tap so that the relay opens when there is an interruption of the added signals from a tamper circuit to the tap over the drop cable. Such a system relies on the power being supplied by the customer and a conversion of the power to produce added signals. However, in some systems, power will be supplied by the provider of services.
Similar schemes with relays at the customer's premises are used for safety reasons to turn off power from the customer's premises to the tap in the event of a break in the drop cable. (See U.S. Pat. No. 4,577,221 issued to Skinner, Sr. et al. and U.S. Pat. No. 4,776,010 issued to Noda et al.)